Triaryl phosphines have proven to be extremely useful in Wittig reactions. During the course of the Wittig reactions, these phosphines are converted to phosphine oxides which are of little use commercially. The number of Wittig type reactions which are being utilized has created a problem with regard to the disposal of the phosphine oxides produced as waste products during this reaction. Furthermore, the phosphine starting materials are rather expensive. The cost of the phosphine starting material coupled with the expense of disposing of the phosphine oxide waste products has proven disadvantageous in commercial Wittig processes. Therefore, there has been a search for an economic means for converting these phosphine oxide waste products back into the phosphine starting materials.
In the past, triaryl phosphine oxides have been converted to the corresponding phosphines by a number of methods such as by treatment with hydrohalosilanes, as described in U.S. Pat. No. 3,261,871, or by first converting the triaryl phosphine oxides into the corresponding phosphine halides and then treating these halides with a reducing metal or phosphorous to form the triaryl phosphine, as described in U.S. Pat. No. 3,405,180 and in U.S. Pat. No. 3,481,988. It has also been disclosed in U.S. Pat. No. 3,280,195 Fritzche et al. Oct. 13, 1966 that phosphine oxides can be reduced to phosphines by utilizing either a reducing metal or hydrogen in the presence of a metal catalyst. In this process, a silicon tetrahalide has been utilized as an auxiliary agent in the reduction.
Furthermore, other specialized reducing agents such as phenyl trihydrosilane, triphenylhydrosilane, methyl polysiloxane, hexachlorodisilane, have been utilized to reduce triaryl phosphine oxides to triaryl phosphines. However, methods for reducing the triaryl phosphine oxides to the corresponding phosphines with conventional reducing agents have met with little success. In many cases, the use of these conventional reducing agents results in the conversion of triaryl phosphine oxide to the corresponding diaryl phosphine. Previous use of catalytic hydrogenation conditions gave ring hydrogenated products rather than deoxygenation. In addition, the use of some conventional reducing agents to reduce these triaryl phosphine oxides requires extreme conditions of heat and pressure in order to carry out the reduction of the triaryl phosphine oxides. Therefore, the use of conventional reducing agents to carry out the reduction of phosphine oxides to the corresponding phosphines has not proven to be entirely satisfactory and commercially feasible.